Vertebral fusion is a common surgical procedure to immobilize two or more vertebrae. In surgical fusion, implants immobilize the vertebrae relative to each other while osteoblastic, or bone-growth inducing material stimulates bone growth between the vertebrae.
Surgeons perform vertebral fusion as a last resort to treat pain or neurological deficits caused by vertebral column abnormalities. Abnormalities indicating fusion include degenerative disc disease, disc herniation, discognetic pain, spinal tumors, vertebral fractures, scoliosis, kyphosis, spondylolisthesis, spondylosis, and Posterior Rami Syndrome. Posterolateral fusion immobilizes the vertebrae with metal implants connected to screws and wires through the pedicles of each vertebrae and bone grafts between the transverse processes. Interbody fusion immobilizes the vertebrae by replacing the cartilaginous disc between the vertebrae with an implant and bone graft. Surgeons generally perform interbody fusion between vertebrae in either the cervical spine or the lumbar spine.
Orthopedic surgeons perform anterior cervical interbody fusion through a small incision in the patient's neck, approaching the cervical spine from the anterior side. Retractors protect vascular and neurological structures on each side of the cervical spine while the surgeon removes the unhealthy cartilage by ablation or some other equivalent process.
Once the surgeon has removed the unhealthy cartilage, he inserts an implant into the disc space between the two cervical vertebrae. Current implants have two shortcomings; the surgeon cannot safely place an implant where it would be most effective; and the surgeon cannot ensure that bone-growth inducing material has made contact with both vertebrae, which is necessary to ensure successful fusion. Both shortcomings are common to implants used in lumbar interbody fusion as well.
Surgeons must generally insert existing implants by gripping the implant along its circumference with forceps and sliding the implant into the disc space. Alternatively, the surgeon may insert the implant by gripping the implant along the top and bottom surfaces and sliding the implant into the disc space. Either method usually requires light hammering with a mallet to fully insert the implant.
These insertion methods have significant drawbacks: gripping the implant along its circumference necessarily requires the implant to have a smaller circumference than the disc space the implant is intended to fill; otherwise the surgeon could not remove the forceps he used to insert the implant. For the same reason, gripping the implant by the top and bottom necessarily requires the surgeon to remove the holder before fully seating the implant. In either case, the surgeon performing the operation will usually use a small mallet and impact tool to tap the implant further into the disc space. While tapping is an effective and controllable means of inserting the implant, tapping also imparts significant loads on the implant that may cause cracking. Furthermore, a surgeon generally cannot pull an implant back if the surgeon taps the implant too far into the disc space, and tapping an implant too far into the disc space poses a danger to the patient's spinal cord; therefore, surgeons usually do not place existing implants in the region of the disc space between the dense cortical bone at the posterior peripheral regions of the vertebrae, even though such placement would provide ideal support. Instead, surgeons generally place existing implants in the center of the disc space. The center of a vertebral body is softer than the periphery of the vertebral body because the center is composed primarily of cancellous bone which is porous; therefore, implants placed in the center of the vertebral body often subside.
Existing implants include one or more bone-growth inducing material containers defined by the circumference of the implant. Bone-growth inducing material stimulates new bone formation in the disc space where the surgeon has removed the unhealthy cartilage, thereby fusing the two adjacent vertebrae. Because existing implants completely circumscribe the bone-growth inducing material containers, a surgeon must pack bone-growth inducing material into those containers before the surgeon inserts the implant into the patient's vacant disc space. During insertion, bone-growth inducing material frequently settles or subsides, or otherwise fails to make contact with each of the vertebra defining the empty disc space. If the bone-growth inducing material does not make contact with both vertebrae, the vertebrae may never actually fuse and the operation will be a failure. Because existing implants completely circumscribes the bone-growth inducing material containers, the surgeon has no way of knowing if the bone-growth inducing material has made contact with each vertebra; nor does the surgeon have an effective means of correcting the situation if not.
Fusion failure is common. Fusion failure typically occurs when the implant subsides into the cancellous bone of the vertebrae, or when the bone-growth inducing material does not properly interface with the vertebrae. Consequently, it would be advantageous if an apparatus existed that is suitable for stabilizing vertebrae, which can be positioned precisely along the posterior peripheral cortical bone regions of two vertebrae, and which a surgeon can pack with bone-growth inducing material after the apparatus is in place.